Brain cholesterol is derived entirely from endogenous synthesis, which is regulated specifically according to the need for the cholesterol incorporation into CNS structures, particularly myelin. Our research objective is to determine how the brain regulates cholesterol synthesis during and after development. We will fist attempt to determine how conversion of squalene and other sterol precursors into cholesterol takes place. We will then measure the relative rates of individual reactions, within the multienzyme system for conversion of squalene to cholesterol. The relative rates of various other reactions in the multienzyme system for converting mevalonic acid to squalene at various stages of development will be determined. We will then investigate the role of enzyme and cofactors in the rate limiting process. Once the rate-limiting reactions have been identified, we will measure their activities separately in glial cell and neuronal cell factions derived from brain of animals at various stages of brain development in order to locate the site of cholesterol synthesis regulation. A reduction in cholesterol synthesis takes place under certain hypomyelinating conditions (e.g. undernutrition, hyperphenylalaninemia, thyroid deficiency, etc.). We will attempt to identify the enzymatic steps that are affected in such conditions and thereby determine whether the condition itself involves further attenuation of a normally rate limiting process or whether a different mechanism of inhibition of cholesterol synthesis takes place.